Paint drying method and apparatus

ABSTRACT

A high-efficiency system for drying solvent-based paint coatings on articles, with minimum fuel requirements and pollution-free emissions, comprises a paint drying oven in which the painted article is dried in a heated atmosphere which is maintained substantially oxygen-free or inert so as to be above the upper explosion limit at all times during the drying process. The combustible solvent vapors which evaporate from the paint coating during the drying process and are contained in the flue gases exhausted from the oven are conveyed into and incinerated under stoichiometric conditions in a rich fume incinerator, containing auxiliary gas-air pilot burners also operating under stoichiometric conditions, to produce substantially oxygen-free nonpolluting inert gaseous combustion products at least a part of which, after cooling in a heat exchanger for heat recovery purposes, is recycled back into the drying oven to supply thereto all or part of the heat required for the paint drying process as well as the essentially inert gas atmosphere required therein. The unused portion of the incinerator combustion products may be vented directly to the atmosphere as nonpolluting emissions.

Hemsath et a1.

Oct. 7, 1975 PAINT DRYING METHOD AND APPARATUS lnventors: Klaus H.Hemsath; Arvind C.

Thekdi, both of Sylvania, Ohio; Frank J. Vereecke, Palmyra, Mich.

[73] Assignee: Midland-Ross Corporation,

Cleveland, Ohio [22] Filed: Feb. 28, 1974 211 Appl. No.: 446,737

[52] US. Cl. .Q 34/26; 34/28; 34/32; 34/72; 23/277 C [51] Int; Cl. F2685/00 [58] Field of Search 34/26, 28, 32, 72, 155, 34/223, 36, 37, 242;23/277 C [56] References Cited UNITED STATES PATENTS 1,432,582 10/1922Will 23/277 C 2,750,680 6/1956 Houdry et a1. 34/36 2,795,054 6/1957Bowen 34/72 3,601,900 8/1971 Erisman et a1 34/26 3,658,482 4/1972 Evanset a1 23/277 C 3,757,427 9/1973 Wilkinson 34/32 3,838,974 10/1974Hemsath et al. 23/277 C n as l 1- Primary Examiner-Carroll B. Dority,Jr. Assistant Examiner-Larry l. Schwartz Attorney, Agent, or FirmFrank.l. Nawalanic [57] ABSTRACT A high-efficiency system for dryingsolvent-based paint coatings on articles, with minimum fuel requirementsand pollution-free emissions, comprises a paint drying oven in which thepainted article is dried in a heated atmosphere which is maintainedsubstantially oxygen-free or inert so as to be above the upper explosionlimit at all times during the drying process. The combustible solventvapors which evaporate from the paint coating during the drying processand are contained in the flue gases exhausted from the oven are conveyedinto and incinerated under stoichiometric conditions in a rich fumeincinerator, containing auxiliary gas-air pilot burners also operatingunder stoichiometric conditions, to produce substantially oxygen-freenonpolluting inert gaseous combustion products at least a part of which,after cooling in a heat exchanger for heat recovery purposes, isrecycled back into the drying oven to supply thereto all or part of theheat required for the paint drying process as well as the essentiallyinert gas atmosphere required therein. The unused portion of theincinerator combustion products may be vented directly to the atmosphereas nonpolluting emissions.

2 Claims, 4 Drawing Figures TO ATMOSPHERE CLEAN GASES 25 US. Patent Oct.7,1975 Sheet 10f3 3,909,953

US, Patent Oct. 7,1975 Sheet20f3 3,909,953

VENT

CLEJAN T GAES T 1 BURNER BURNER GAS GAS AIR AIR |o PUMP AIR V CLEANGASES 70 y t VENT II 56 5e BURNER BURNER sAs GAS 7 AIR M AIR PUMP AlR-HH l 54 II 45 T 582 e? MgTTAllC L TRI PREPARATION U.S. Patent AMBIENTATMOSPHERE Oct. 7,1975

Sheet 3 of 3 OVEN ATMOSPHE R E PAINT DRYING METHOD AND APPARATUSBACKGROUND OF THE INVENTION The present invention relates, in general,to method and apparatus for carrying out heating operations whichrelease flammable solvent vapors, and more par ticularly to method andapparatus for drying solventbased paint coatings on articles of variousform.

Solvent-based paints normally contain from 40 to 60 percent organicsolvents usually in the form of aromatic hydrocarbon compounds, such asbenzene, toluene, xylene and other high-flash napthas which are derivedfrom coal-tar distillates and which are evaporated from the paintcoating during the drying process. Because these solvent vapors arehighly flammable and can form explosive mixtures when mixed with air,they represent potential fire and explosion hazards. For this reason,insurance and fire prevention regulations re quire that for safeoperation of drying ovens open to the atmosphere the solvent vaporconcentration in the oven exhaust gases must be lower than 25 percent ofthe lower explosion limit (L.E.L.) thereof. To comply with theseregulations therefore, and independent of the drying process heatdemand, the solvent vapors evolved in such ovens must, before theirexhaustion therefrom, be diluted to 25 percent or less of their lowerexplosion limit. In conventional paint drying ovens, this dilution isobtained by introducing into the oven at least 10,000 cu. ft. of air pergallon of evaporated solvent. Prior to the enactment of recent pollutioncontrol regulations, these solvent-air mixtures con taining undesirablepollutants were discharged directly into the atmosphere.

The heat required to operate such a conventional paint drying system isused not only for heating the paint coated article to the required paintdrying temperature, usually from 200 to 600F., but also is used up inheating the dilution air to the oven exhaust temperature. The added heatenergy thus expended in heating the required large volume of dilutionair represents a considerable proportion of the total fuel requirementfor the system and therefore a considerable added operating expense.

Because of the elevated levels of the pollutants contained in thesolvent-air mixtures exhausted from conventional paint drying ovens,recently enacted governmental pollution control regulations no longerpermit the emission of these mixtures directly into the atmosphere. Tocomply with these air pollution control regulations, therefore, it hasbeen necessary to equip paint drying ovens with some type of pollutionabatement equipment, the most common form of which has been a thermalfume incinerator for incinerating the exhaust gases from the paintdrying oven. Such thermal incineration of these oven exhaust gasesnormally requires temperatures between 1,250F. and l,450F. In a typicalfume incinerator, the fumes are mixed with auxiliary fuel, e.g., naturalgas, and the mixture temperature is raised to l ,250l ,450F. to causethe incineration of the fumes) With proper design of the incinerator,the gaseous products of combustion are essentially clean, i.e., free ofair pollutants. The auxiliary fuel requiremerits for the incinerationprocess depend on the exhaust fume temperature and on the initial fumeloading. In most paint drying applications employing such exhaust gasincineration, however the fuel consumption for the paint dryingoperation increases more than twice compared to that used in theconventional paint drying system not equipped with a fume incinerator.

To reduce the fuel consumption of such incineratorequipped paint dryingsystems, it has been proposed to include a fume preheater in which theexhaust gases from the incinerator are used to heat the fumes and raisetheir temperature before entering the incinerator. However, the additionof such a fume preheater increases the capital cost for the system by anamount comparable to or higher than the cost of the incinerator itself.In many cases, economic considerations make it impractical to raise thepreheater efficiency beyond to percent, with the result that theadditional fuel consumption for the operation of the incinerator isstill substantial. A further proposal for reducing the fuel consumptionstill further has been to employ a liquidto-gas heat exchanger torecover additional heat from the exhaust gases leaving the fumepreheater. But such a system is considerably more expensive and its useis limited to very large installations. Even with such an elaborate heatrecovery system, however, the fuel consumption is higher than the simpleconventional system without fume incineration equipment.

Many small and medium scale paint drying installations cannot justifyfrom an economic standpoint the use of a complete heat recovery system,with the result that they have to use considerably more fuel than in thepast in order to operate the required pollution abatement equipment orincinerator. Moreover, the present fuel supply shortages and thecontinuously rising fuel costs are threatening the paint drying industrywith higher production costs, insufficient fuel allocations andproduction cutbacks or interruptions. As alternatives to the use ofsolvent-based paints, the use of powder coatings and water-based paintsis being considered as a way of reducing or eliminating air pollutingemissions and hence the additional fuel consumption in the incinerator,but the use of any one of these two alternatives still requiressubstantial fuel usage in the drying or heating oven. Moreover, so faras known, none of these alternatives has been tried as yet on a largescale so that their ultimate use in large scale coating operations maynot occur for a considerable time.

SUMMARY OF THE INVENTION As mentioned hereinabove, conventionalsolventbased paints contain from 40 to 60 percent or higher of organicsolvents which are highly inflammable and have heating value of 120,000to 140,000 B.t.u. per gallon of solvent. These paints are customarilyapplied as a thin coating or film on the article or object to bepainted, the film thickness in most applications being of the order of0.5 to 2 mils or thereabouts. During the drying process most of thesesolvents are evaporated, and the pigments of the paint form a protectivecoating on the article. The heating value of the solvent vaporsevaporated during the paint drying process is usually many times morethan the fuel requirements in a typical paint drying oven. Accordingly,a paint drying system designed to take advantage of this heat energy ofthe solvent vapors, while maintaining the basic requirements of safety,pollution free exhaust emissions and unchanged coating or productquality, will provide a drastic reduction in the fuel demand of thepaint drying industry.

In conventional paint drying systems which operate with oven atmospheresbelow the lower explosion limit,

the dilution air which must be added to the solvent vapors beforeexhausting them from the oven in order to comply with safety regulationsuses up a major portion of the total heat requirement for the system.Moreover. bringing the exhaust gases to an incineration temperature ofl,250 to 1,450F. to comply with air pollution regulations can more thandouble the heat requirements of systems not provided with oven exhaustincineration means. By eliminating or reducing the dilution air withoutendangering the safety requirements of the paint drying system, a largepart of the intrinsic heat energy in the solvent vapors released fromthe paint on drying can be utilized for heating the drying oven and thepainted article therein to be dried. In the design of such a system,however, considerations relating to safety and perfect combustionrequire an understanding of the fundamentals of the flammabilitycharacteristics of gas mixtures, such as are formed in paint dryingovens, by reference to the established flammability curves therefor.These curves show that mixtures of common flammable gases includinghydrogen, carbon monoxide, hydrocarbons and vapors of common paintsolvents, with inert gases such as nitrogen, carbon dioxide and watervapor, are nonflammable if the oxygen concentration in the mixture isless than about percent. Accordingly, and as an alternative to the useof large quantities of dilution air to control potential explosionhazards, the gas-solvent vapor mixture formed in paint drying ovensduring the drying operation will remain nonexplosive if an atmospherewith a low oxygen content, or an inert atmosphere with essentially nooxygen content, is maintained in the oven during the solvent evaporationfrom the paint coating In this way, it is not necessary to use anydilution air at all because the gas-solvent mixture is outside andconsiderably away from the flammability region, i.e., it is above theupper explosion limit (U.E.L.) thereof. While, from a safety standpoint,the volume ratio of inert gases to the solvent vapors may in such gas beany value, in most instances the required volume of inert gas per gallonof evaporated solvent is much less than the dilution air required inconventional paint drying systems.

The nonexplosive gas-solvent mixture or rich fumes exhausted from thedrying oven can be utilized to supply all the heat required for thepaint drying operation by incinerating the oven exhaust gases, understoichiometric conditions, in an incinerator to convert them intooff-gases of substantially inert character and high temperature, e.g.,from 1,400 to 1,650F. or thereabouts. Because of their inert character,these hot exhaust gases from the incinerator then can be recycled, aftercooling, back into the paint drying oven to not only supply thereto all,or if desired only a part of the heat required for the paint dryingoperation but to also supply thereto the required amount of inert gasesto assure the maintenance in the oven of a gas-solvent vapor mixtureabove the upper explosion limit thereof at all times during the paintdrying operation. Since the volume and heat energy of the exhaust gasesgenerated in the incinerator are substantially greater than thatrequired for the paint drying operation in the oven, only a part of thetotal volume of these exhaust gases need be recycled back into the oven.In view of their inert and therefore nonpolluting character, the otherpart of these exhaust gases can be vented directly to the atmospherewithout any polluting effect thereon. The portion of the hot incineratoroff-gases recycled to the oven can be cooled to the lowered temperaturerequired for the paint drying operation in the oven in a suitable heatrecovery system or heat exchanger such as a water or an air heater, or alow pressure steam boiler, or a metal preparation section of the articlecoating line. Thus, the incinerator is used as a means of energyrecovery and serves the multi-purpose function of an inert gasgenerator, a heat generator or burner, and as pollution abatementequipment. Moreover, the incinerator does not require any auxiliary fuelfor its normal operation except for the very small amount needed tooperate pilot burners to maintain the required ignition conditions inthe incinerator for assuring the complete combustion of the solventvapors contained in the oven exhaust gas mixture introduced into theincinerator.

It is an object of the invention, therefore, to provide a system fordrying solvent-based paint and other solvent-containing coatings whichis of increased thermal efficiency.

Another object of the invention is to provide a system for drying suchsolvent-based coatings which not only is of improved thermal efficiencybut also is characterized by pollution-free emissions.

Still another object of the invention is to provide a system for dryingsolvent-based coatings which is substantially self-supporting in respectto its fuel energy requirements.

A further object of the invention is to provide a system for drying suchsolvent-based coatings in a substantially oxygen-free or low oxygencontent inert atmosphere maintained well above the upper explosion limitat all times during the drying operation and complying with standardfire safety regulations therefor.

A still further object of the invention is to provide a system fordrying such solvent-based coatings in which the latent heat energy inthe solvent vapors evaporated from the coating during the dryingoperation is used to supply all or part of the heat required for suchoperation.

Another object of the invention is to provide a system for drying suchsolvent-based coatings in which the solvent vapors evaporated from thecoating during the drying operation are incinerated in a manner toproduce substantially oxygen-free or low oxygen content inert gaseouscombustion products for recycling back into the drying oven to supply aninert atmosphere therein.

Still another object of the invention is to provide a system for dryingsuch solvent-based coatings which does not require the addition of anydilution air to the gases exhausted from the drying oven during thedrying operation in order to maintain them outside their explosionlimits so as to comply with standard fire safety regulations.

A further object of the invention is to provide apparatus foreffectively carrying out the drying of solventbased paint and other suchcoatings in accordance with the above referred to systems.

Briefly stated, in accordance with one aspect of the invention, thedrying of solvent-based paint and other such type coatings on articlesare carried out in a drying oven in which a heated atmosphere ismaintained of substantially oxygen-free or low oxygen content inertcharacter so as to be above the upper explosion limit at all timesduring the drying operation. The solvent vapors'evaporated from thecoating and mixed with the inert gases exhausted from the drying ovenare incinerated under stoichiometric conditions in an incinerator toform substantially oxygen-free or low oxygen content nonpolluting inertoff-gases at elevated temperatures of l,400 to l,650F. which may bevented directly to the atmosphere as nonpolluting emissions from thesystem. At least a part of the off-gases, after cooling down in asuitable heat recovery system to the required temperatures of from 200to 600F. for the coating drying operation, is recycled back into thedrying oven to not only supply thereto all or part of the heat requiredby the oven for the drying operation but also to supply the inertatmosphere for the oven.

In accordance with a further aspect of the invention, the only auxiliaryfuel requirement of the coating drying system according to the inventionduring the normal operation thereof is the small amount required for theoperation of pilot burners within the incinerator combustion chamber tomaintain the required ignition conditions therein for assuring thecomplete combustion of the solvent vapors introduced thereinto from theoven.

Further objects and advantages of the invention will appear from thefollowing detailed description of species thereof and from theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a vertical cross-sectionalview, partly schematic, of a paint drying system according to theinvention;

FIG. 2 is a flow chart of the basic system shown in FIG. 1;

FIG. 3 is a flow chart of a modified paint drying system according tothe invention; and

FIG. 4 is a vertical cross-sectional view, on an enlarged scale, of oneof the aerodynamic seals which are located at the article inlet andoutlet openings in the drying oven shown in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the drawings, the inventionis therein illustrated as embodied in a system for the continuous dryingof solvent-based paint coatings on metallic strip stock such as metalliccoiled strip. It should be understood, how ever, that the invention isapplicable as well to the drying of solvent-based paint coatings onarticles of various other types such as, for example, automobile bodies,metal cabinets for household appliances, etc., and also to other heatingprocesses in which high B.t.u. va pors are released as in chemicalcoating, pyrolysis of carbonaceous material, carbon baking operations,and resin coating processes.

Referring to FIG. 1, there is shown at an oven in which heatingoperations are to be carried out which result in the release offlammable vapors of high B.t.u. content, in the particular caseillustrated the heating of metallic strip stock material 11 such assteel or aluminum strip coated with a layer or film 12 (FIG. 4) of asolvent-based paint on one or both sides. In the exemplary continuousmetallic strip paint coating process as shown in FIG. 1, the metallicstrip material 11 is uncoiled from a supply roll or coil 13 thereof asby means of a pair of feed rolls 14 between which the strip 11 passesand from which it is then dipped or immersed in a bath 15 ofsolvent-based paint contained in a coating tank 16 to form the coating12 on the strip. It should be understood, however, that the coating 12of paint on the strip material 11 could be .applied thereto by methodsother than the illustrated dip-coating method, e.g., by spraying,rolling, etc. From the coating tank 16 the coated strip 11 passesbetween and is supported by a pair of squeegee rolls 17 which squeezethe excess coating material from the strip so as to leave the de siredthickness film or coating 12 thereon which, in the case of conventionalpaint coatings, is usually from 0.5 to 2.0 mil thickness. The coatedstrip 11 passes from the squeegee rolls 17 into the interior or dryingchamber 18 of the oven 10 through an inlet or entrance opening 19 in anend wall 20 of the oven and then, after the drying of the coating 12 onthe strip 11, out of the oven chamber 18 through an outlet or exitopening 21 in the other end wall 22 of the oven. The emerging strip 11from the oven 10 and on which the coating 12 is now dried passes betweenand is supported by a pair of idler rolls 23 and is then coiled on atake-up roll 24. The oven chamber 18 is supplied with a heatedatmosphere at the required temperature for the particular dryingoperation or other heating operation to be carried out therein, e.g., atemperature in the range of from about 200 to 600F. in the case ofconventional solvent-based paint coatings 12 to be dried.

As mentioned hereinabove, where any heating operation carried out in anoxygen-containing atmosphere in an oven results in the evolution ofvapors in the oven which are highly flammable and of explosivecharacter, such as the solvent vapors which are evaporated fromsolvent-based paint coatings 12 during the drying thereof, theapplicable insurance and fire regulations require that for safeoperation of the oven the solvent vapors evolved therein must, beforethe exhausting thereof from the oven, be diluted to 25 percent of, i.e.,well below, the lower explosion limit (L.E.L.) of the solventvapor-containing gas mixture formed in the oven during the heatingoperation. In conventional paint drying ovens, this dilution is obtainedby introducing into the oven 10,000 cu. ft. of air per gallon ofevaporated solvent. Because this dilution air introduced into the ovenmust, of necessity, be also heated to the temperature required for theheating operation to be performed in the oven, the added heat energythus expended in heating the dilution air represents a considerableproportion of the total heating fuel requirement of the oven andtherefore a considerable added operating expense.

In accordance with the invention, the need for and added expense ofheating such dilution air is entirely eliminated by carrying out thepaint drying or other heating operation to be performed in the oven in asubstantially oxygen-free or low oxygen content inert atmosphere whichis above the upper explosion limit (U.E.L.) of the particular gasmixture formed in the oven during the heating operation instead of belowthe lower explosion limit (L.E.L.) of such gas mixture as has beencustomary practice heretofore. The flammability characteristics of anygas mixture can be determined by reference to the establishedflammability curves for various gas mixtures. These curves show that amixture of common flammable gases including hydrogen, carbon monoxide,hydrocarbons and vapors of common solvents such as employed inconventional solvent-based paints, with inert gases such as nitrogen,carbon dioxide and water vapor, is nonflammable if the oxygenconcentration in the mixture is less than 5 percent. For the purposes ofthe invention, therefore, the oven 10 is supplied with a substantiallyinert gas atmosphere having an oxygen concentration of less than percentand preferably less than 2 percent. The inert gas atmosphere can besupplied to the oven from a separate source of supply thereof.Preferably. however. and to realize the fullest benefits of theinvention from an economic standpoint, the inert gas atmosphere suppliedto the oven 10, as well as the heat required therein for the paintdrying or other solvent vaporizing heating operation conducted in theoven, is supplied in the manner shown by the flow chart in FIG. 2, bythe exhaust or off-gases from an incinerator in which the mixture ofinert gas and solvent vapor exhausted from the oven 10 is incinerated ina manner as described hereinafter. At least a part of these off-gases,after cooling to the temperature required for the paint drying or otherheating operation to be performed in the oven 10, is recycled back intothe oven to supply the inert gas atmosphere and all or part of the heatrequired therein. Thus, the paint drying or other solvent vaporizingsystem according to the invention may be substantially self-supportingin respect to its fuel energy requirements.

To maintain the desired low oxygen concentration in the oven atmosphereat all times during the continuance of the paint drying or other heatingoperation carried out in the oven 10, it is necessary to isolate thedrying chamber 18 of the oven and thus the atmosphere therein from theambient atmosphere to prevent leakage of air into the oven. To this end,the walls of the oven 10 are made of proper design to be substantiallyair tight in themselves. Thus, as shown in FIG. 1, the end walls 20, 22,side walls 26 and top and bottom walls 27 and 28, respectively, may beformed as a sheet metal shell or casing 29, the inner side of which maybe prov vided with a lining 30 of a suitable heat refractory materialsuch as, for example, ceramic fiber type block insulation. Furtherassurance against leakage of ambient air into the oven chamber 18 isafforded by maintaining the inert gas atmosphere therein at a slightoverpressure of, for example, 0.05 inches w.c. (water column) or so.Also, where, as in the particular paint drying operation illustrated,the work to be processed in the oven 10 is continuously advanced intoand out of the oven through inlet and outlet openings 19 and 21 therein,the isolation of the oven chamber 18 from the outside atmosphererequires that these openings 19, 21 also be sealed off from the outsideatmosphere. For this purpose, the oven 10 may be formed at its inlet andoutlet openings 19, 21 with aerodynamic seals 31 of suitable formv Asshown more particularly in FIG. 4, where the work to be processed in theoven 10 is in the fomi of a continuous strip such as the paint coatedmetallic strip material 11, or in the form of articles conveyed into andout of the oven on a conveyor belt or similar such means, theaerodynamic seals 31 may in such case be formed by curtains 0r screens32 of inert gas directed normal to and against the opposite flat sidesof the strip material 11 or conveyor means immediately outside each ofthe openings 19 and 21 in the oven. For the gas screens 32 to beeffective to seal off the openings 19, 21 from the outside atmosphere,the inlet and outlet openings 19, 21 in the oven walls 20, 22 may be inthe form of narrow rectangular shaped slots about 6 inches high, forexample, and wide enough to accept.

with a slight clearance at each side, the maximum width strip material11 or conveyor to be advanced through the openings. The inert gasscreens 32 may be directed against the advancing strip 11 or workcarrying conveyor from respective pairs of plenums 33 and 34 mounted onthe outside of the oven end walls 20, 22 at locations immediately aboveand below the inlet and outlet openings 19 and 21 so as to be positionedon Opposite sides of the advancing strip 11. The plenums 33 and 34 areof an extent conforming to the width of the inlet and outlet openings19, 21 in the oven walls 20, 22 and they are provided with opposed flatplenum outlet plates 35 of perforated form, and disposed parallel to andfacing the strip 11, to provide outlet openings for the inert gasforming the gas stream curtains 32. The plenum outlet plates 35 may havea dimension longitudinally of the advancing strip 11 suitably around 6inches or so, and they may be spaced apart around 9 inches orthereabouts so as to be each spaced approximately the same distance ofaround 4% inches or so from their respective side of the advancing strip11. The perforated outlet plates 35 may have small diameter gasdischarge or outlet holes (not shown), the centers of which are spacedapart a distance of two outlet hole diameters and are arranged instaggered pattern to provide approximately 33 percent open surface areain the plates 35. To prevent leakage of the sealing-off streams 32 atthe side ends of the space between the plenums 33, 34, end plates 36bridging the plenums at their respective ends and closing off the spacetherebetween are provided on the plenums.

The inert gas for the plenums 33, 34 is supplied thereto by respectivegas inlets or ducts 37 which open into the plenum chambers and areconnected to a suitable source of inert gas which, in accordance withthe invention, may be the exhaust gases from the incinerator 25.Sufficient inlets 37 to each of the plenums 33, 34 should be provided toprevent uneven gas distribution therein. Satisfactory results areobtained in this respect with one side inlet 37 per foot of extent ofthe plenum along the width of the respective slot-shaped inlet or outletopening 19, 21. With the particular dimensional openings 19, 21 andplenums 33, 34 as described above, the quantity of inert gas required tobe supplied to the plenums 33, 34 to produce an effective aerodynamicseal 31 at the oven openings 19, 21 is much less than the off-gasby-products from the incinerator 25. A sufficient excess of such inertofi-gases from the incinerator 25 is therefore available at all times tosupply the needed atmosphere for the plenums 33, 34 to maintain theaerodynamic seals 31 at the inlet and outlet openings 19, 21 of the oven10.

In the operation of the aerodynamic seals 31, the inert gas streams 32discharged from the plenums 33, 34 in directions normal to the advancingstrip 1 1, after impinging on the strip, then turn and proceed along andmore or less parallel to the strip 11 as indicated by the arrows in FIG.4, part of the gas stream flowing inwardly into the oven 10 and theother part flowing 0utwardly thereof. This flow pattern operates toeffectively prevent outside air from entering the oven 10 through theoven inlet and outlet openings 19 and 21 as well as to prevent the inertoven atmosphere from escaping to the outside atmosphere through theseopenings.

For proper operation of the illustrated paint drying operation. thepaint coated strip 11 is advanced through the heated oven chamber 18 ata rate, depen dent upon the specific operating temperature of the ovenatmosphere and the thickness and particular composition of the paintcoatings 12, such as will assure the substantially complete drying ofthe paint coatings 12 on the strip before the exiting thereof from theoven chamber 18 through the oven outlet opening 21. During this paintdrying operation, the solvents in the paint coatings 12 are evaporatedtherefrom and the resulting solvent vapors mix with the inert gasatmosphere maintained in the oven chamber 18. Because this gassolventvapor mixture is maintained below approximately percent oxygen contentat all times during the continuance of the paint drying operation, ittherefore is above the upper explosion limit and outside theflammability region of the flammable components thereof and thus can bedirectly exhausted safely from the oven without the need of diluting itwith any air whatsoever to comply with insurance and fire regulationssuch as has been customary practice heretofore with prior conventionalpaint drying systems. As shown, the inert gas-solvent vapor mixture isexhausted from the oven chamber 18, preferably at a region of the roofthereof near or at the back or outlet end of the oven, through anexhaust duct 38 (FIG. 1) and as shown by the flow line 38a in the flowcharts of FIGS. 2 and 3. The gas solvent vapor mixture is suitablyexhausted from the oven chamber 18, as by means of an exhaust pump orfan 39 in duct 38, at the required rate to maintain the solvent vaporconcentration in the oven atmosphere at the desired level. In thisregard, while from the viewpoint of safety requirements the volume ratioof inert gases to the solvent vapors in the oven atmosphere can assumeany value, the vapor condensation and other considerations may requirethat this ratio be maintained below a definite numerical value. In mostcases, however, the required volume of inert gas per gallon of theevaporated solvent is in any event many times less than the dilution airrequired in conventional solventbased paint drying systems.

The oven atmosphere may be maintained at the aforementioned slightpositive or overpressure of around 0.05 inch w.c. by a conventionalpressure controller 40 (FIG. 1) which is actuated by a pressuremeasuring unit 41 in the oven chamber 18. The pressure controller 40 isarranged to control the position of either a damper 42 in the gasexhaust flue 38 from the oven chamber 18 or a damper 43 in the inert gassupply intake 44 therefor.

The inert gas-solvent vapor mixture or so-called rich fume exhaustedfrom the oven 10 is, in accordance with a further aspect of theinvention, introduced into a special design incinerator 25, known as arich fume incinerator, where the rich fume is incinerated at atemperature of at least I,400F. and under approximately stoichiometricconditions to convert the rich fume into clean, i.e., nonpolluting,inert gaseous products of combustion or off-gases such as nitrogen,carbon dioxide and water vapor and having an oxygen concentration ofless than about 5 percent, preferably about 2 percent in the particularpaint drying process illustrated, such as are permissible to vent to theatmosphere under existing pollution control regulations. For thepurposes of the invention, the rich fume incinerator may be of the typedescribed and claimed in US. application Ser. No. 274,406, K. H. Hemsathand A. C. Thekdi, filed July 24, 1972 and assigned to the assignee ofthe present application. As therein disclosed, the incinerator 25comprises in general a stack 45 (FIG. 1) which is lined on the insidewith refractory bricks in order to preserve heat and into which the richfume exhausted from the oven 10 is discharged. Disposed about and abovethe generally cylindrical stack 45 is a housing 46 having a lowerchamber 47 and an upper or combustion chamber 48, the housing also beinglined with refractory material. The lower chamber 47 has a restrictedcylindrical passageway 49 extending upwardly and opening into the upperchamber 48 and terminating a short distance above the upper end of thestack 45 with which it is concentric and which it closely surrounds toform therebetween a narrow annular air supply channelway 50. Thecombustion chamber 48 is abruptly widened at the upper end of therestricted passageway 49, as by means of the step shoulder 51 in thewall of the chamber 48, to provide flame stabilization means forpromoting the complete combustion of the rich fume and pump air mixturewithin the combustion chamber. The lower chamber 47 also has an opening53 for an air supply pipe 54 provided with a manual valve 55. The airsupply pipe 54 provides the required quantity of pump air for theapproximate stoichiometric combustion of the rich fume in the upper orcombustion chamber 48 of the housing 46. The pump air directed into thelower end of the combustion chamber 48 from the restricted annularchannelway 50 is discharged into the combustion chamber at a velocitygreater than that of the rich fume exhaustdischarged thereinto from thestack and thus acts as an air pump to aspirate into, and enhance thedischarge or emission of the rich fume from the stack 45 into thecombustion chamber. Additionally, the pump air from the annularchannelway 50 promotes uniform mixing thereof with the rich fumedischarged from the stack 45 so as to better assure the completecombustion in the combustion chamber 48 of all the combustibles in therich fume introduced thereinto from the oven chamber 18.

The upper or combustion chamber 48 of the incinerator 25 is provided atits lower end, at the region of the intermixing therein of the pump airfrom channelway 50 with the rich fume from the stack 45, with a numberof pilot or auxiliary gas burners 56 directed tangentially to thecombustion chamber and operated with air and natural gas mixturesproportioned for approximate stoichiometric combustion thereof. Thepilot burners 56 provide a pilot flame or ignition source in thecombustion chamber 48 and assist in maintaining the com bustion chamberat the required temperature of at least about 1,400F., and preferablyabout l,650F., for assuring the complete combustion or incinerationtherein of the combustibles in the rich fume discharged thereinto fromthe stack 45. The supply of combustion air and gas to the pilot burners56 is preset by manual adjustment of valves 57 and 58 in the air line 59and gas line 60, respectively, to provide the ignition source for therich fume-pump air mixture introduced into the incinerator. Since thesolvent concentration in the exhaust gases or rich fumes from the oven10 is, in the process according to the invention, considerably higherthan that in conventional paint drying systems because of thenonrequirement for the addition of any dilution air to the oven exhaustgases, the heat released by the solvent vapor combustion in theincinerator 25 is ordinarily high enough to itself bring the fume-airmixture in the combustion chamber 48 to the required incinerationtemperature of from l,400 to 1,650F. Thus, for the normal operation ofthe paint drying system according to the invention, the incinerator 25does not require any auxiliary fuel except for the very small amount fora pilot flame from the burners 56. The fuel requirement of the oven 10is additionally lowered, compared to that required by conventionalsystems, because the inert gas heat load is much less than what isrequired for the conventional dilution air paint drying systems.

To permit the use of the off-gases from the incinerator 25 as the sourceof supply for the low oxygen content atmosphere, i.e., no more thanabout percent oxygen content, required to be maintained in the ovenchamber 18 for the practice of the invention, it is therefore necessarythat these incinerator off-gases contain not more than about 5 percentoxygen. Since the pilot burners 56 in the incinerator 25 are operated atapproximately stoichiometric air-fuel ratio, they release little if anyoxygen into the off-gases from the incinerator. However, the solventvapors in the rich fumes in troduced into the incinerator 25 from thedrying oven need oxygen for their combustion in the incinerator, thisoxygen being supplied by the pump air introduced into the lower chamber47 of the incinerator. The solvent concentration in the gas mixtureintroduced into the incinerator combustion chamber 48 is maintained insuch a way that the heat generated by the perfect, i.e., stoichiometriccombustion of the solvent vapors in this mixture is enough to raise itto at least the l,400 to l,650F. temperature required to be maintainedin the combustion chamber for the complete combustion of the solventvapors. To assure that the off-gases from the incinerator 25 normallycontain no more than about 5 percent and preferably around 2 percentoxygen content, the pump air supply to the incinerator is preset bymanual valve 55 in accordance with the de sign solvent vapor emissionsfrom the oven 10.

For maintaining the oxygen content in the flue or offgases from theincinerator 25 at the desired low set value, e.g., 2 percent, theelectrical signal from a temperature controller means can be used forthis purpose. The temperature controller means may comprise athermocouple 61 in the combustion chamber 48 connected by a lead 62 to atemperature measuring and recording instrument 63. The signal from thetemperature measuring and recording instrument 63 is fed through a lead64 to a fuel controller unit 65 which then actuates the valve 66 in anauxiliary gas inlet 67 to the rich fume stack 45. As previously stated,the pump air supply 54 to the incinerator 25 is maintained constant fora given type of load in the oven 10. This constant flow of pump air ispre-established by design conditions of solvent vapor emissions from theoven 10. When the solvent concentration in the rich fume from the oven10 decreases, the temperature of the oven 10 also drops withcorrespondingly less solvent evaporated therein, causing the oxygenconcentration in the incinerator flue products to rise. When thisoccurs, the signal from the temperature controller means 61- 63 willcause the fuel controller unit 65 to open the auxiliary gas supply valve66 so as to add more gas fuel and thus more heat to the incinerator 25by its combustion with the available excess oxygen therein and so reducethe oxygen content in the incinerator flue products. Since the heatrelease from the auxiliary fuel replacing the drop-off in the amount ofthe solvent vapors in the rich fumes, and the oxygen requirement for theoxidation or combustion of the solvent vapors and auxiliary fuel are alllinearly proportional, the rate of temperature rise in the incineratorcombustion chamber 48 and corresponding flue products therefor will besubstantially the same. A conventional type oxygen indicator 68 with ahigh-low limit, e.g., O to 5.0 percent oxygen content actuating range,may be used for the purpose of providing an electrical signal foractuating an alarm when the oxygen concentration in the incinerator fluegases drops below the set oxygen concentration limit, in which case anoperator will then investigate the possible problems in the system, suchas insufficient pump air. When the oxygen indicator 68 senses an oxygenconcentration above its high limit oxygen concentration setting, theentire system is then shut down by the automatic closure of all systememergency shut-off gas valves (not shown) which are actuated by theoxygen indicator 68. In this case, the entire oven, incinerator and gasrecirculation system is automatically purged with an adequate supply ofan inert atmosphere such as nitrogen for safety against explosionhazard. Under normal operating conditions, however, including changeoverof the paint coated strip 11 or breakage thereof, the fuel supply to theauxiliary gas inlet 67 will be enough to reduce the oxygen percent inthe incinerator flue gases below the maximum set limit.

The incineration of the rich fumes from the oven 10 in the incinerator25 in the manner according to the invention as described above resultsin the formation of so-called clean incinerator flue or off-gases suchas nitrogen, carbon dioxide and water vapor which are of inert andnonpolluting character and which therefore are permissible to ventdirectly to the atmosphere under present pollution control regulations.In accordance with a further aspect of the invention, however, theseoff-gases or by-products from the incinerator 25, because of their inertand substantially oxygen-free composition, i.e., containing no more thanabout 5.0, and preferably around 2.0 percent oxygen in this particularcase, are recycled at least in part back into the oven chamber 18 byrecycling means comprising exhaust flue 69 to supply to the oven chamberthe inert gas atmosphere as well as the heat required for the conduct ofthe particular solvent-evaporating operation to be carried out therein,such as the illustrated solventbased paint drying operation. Althoughthe recycled portion of the off-gases may be enough to supply only apart of the total heat and total inert gas requirement of the oven 10,it is preferable from economic considerations to employ a sufficientamount of the off-gases from the incinerator to supply all of the totalheat and inert gas requirement of the oven 10.

Inasmuch as the flue or off-gases exiting from the incinerator 25through flue 69 are normally at a temperature in excess of 1,400F. whichis considerably above the 450 to 600F. temperature required in the ovenchamber 18 for the paint drying or other solventevaporating operationcarried out therein, the off-gases to be recycled back into the oven 10therefore must first be cooled down to such lowered oven operatingtemperature as by passage of the gases through one or more suitable heatrecovery systems or heat exchangers 70 of the recycling means, such as awater or air heater, or a low pressure steam boiler, or a heater for ametal preparation solution employed to prepare the metal strip 11 forthe application of the paint coating 12 thereto. The heat thus recoveredin the heat exchanger 70 can be advantageously employed, as a furthereconomic benefit of the process comprising the invention, for varioussupplementary heating operations.

The flue gases cooled in the heat exchanger 70 are exhausted therefromand discharged, as by a recirculating fan 71, into the inert gas intake44 of the oven to thereby supply the inert gas atmosphere for the ovenchamber 18. As shown in FIG. 1, the gas intake 44 may be formed with amanifold 72 from which the inert gases from the incinerator 25 aredistributed to, and introduced into the oven chamber 18 through a seriesof gas inlet or supply openings 73 in the side wall 26 of the oven 10.Before their introduction into the oven 10, however, the cooled fluegases from the heat exchanger 70 are preferably first mixed with some ofthe existing inert gas atmosphere in the oven chamber 18 for the purposeof assuring better temperature homogeneity of the inert gas atmosphereintroduced into the oven chamber 18 with that present therein during theoperation of the system. This intermixing may be achieved by exhaustinga portion of the inert gas atmosphere from the oven chamber 18 throughan exhaust opening 74 in the side wall 26 of the oven 10 and conveyingthe so exhausted oven atmosphere through a duct 75 into a suitablemixing means 76 where it is mixed with the cooled inert flue gases fromthe heat exchanger 70. The resulting gas mixture is then circulated bythe fan 71 into the oven chamber 18 through the manifold 72 and gassupply openings 73 in the oven wall 26. The portion of the cooled inertoff-gases leaving the heat exchanger 70 that is not recycled back intothe oven 10 may be supplied to the plenums 33, 34 for the production ofthe aerodynamic seals 31 at the article inlet and outlet openings 19 and21 of the oven 10. Manually set dampers 77 and 78 in the respectiveducts 79 and 80 which supply the recirculated cooled inert gases fromthe heat exchanger 70 to the oven 10 and aerodynamic seals 31,respectively, are employed to properly balance the flow of the off-gasesfrom the incinerator 25 to the seals 31, to the atmosphere, and recycledto the oven 10.

FIG. 3 illustrates a modification of the invention wherein all theoff-gases from the incinerator 25 are first passed through one or moreheat exchangers 70 before the unused portion thereof not employed forthe heat requirements of the oven 18, or for the seals 31, is vented tothe atmosphere. In this way, heat recovery from all the incineratoroff-gases may be obtained instead of just from the unvented portionthereof. In the particular case illustrated, the heat recovered in theheat exchanger 70 is shown as being conducted through a duct 81 to ametallic strip preparation section 82 of the illustrated strip paintcoating line, such as for heating a metal preparation solution used toprepare the metallic strip 11 for the application of the paint coatings12 thereto. Thus, a further reduction in the total heat required for thepaint coating and drying system is obtained.

In the start-up of the paint drying or other solvent vapor releasingprocess according to the invention, the oven 10 must be purged ofatmospheric air. For this purpose, any inert gaseous medium such as, forexample, nitrogen, steam, or combustion products from burners 56 andauxiliary fuel 67, can be used to purge the oven chamber 18. An oxygenindicating device should be used to indicate when the oxygen content ofthe oven atmosphere is less than the maximum permissible concentrationthereof of no more than 5.0 percent, e.g., about 2.0 percent, beforestarting the introduction into the oven 10 of the work to be processedtherein. The oven seals 31 are activated before the metallic strip 1 1or other work conveyor is started into the oven. The burners 56 and theauxiliary fuel 67 in the incinerator 25 will supply the heat forbringing the system to the required oven and incinerator temperaturesand the inert atmosphere to the oven seals 31. Auxiliary fuel 67 will berequired during start-up due to the addition to the incinerator 25 ofthe constant and preset amount of pump air through the supply pipe 54provided with the manual valve 55. The incinerator temperaturecontroller means 6163 will adjust the fuel input rate to the auxiliaryfuel inlet 67. Once the operational conditions are settled, the metallicstrip 1 1 or the work conveyor is started into the oven 10.

The economic advantages of the paint drying system comprising theinvention will be readily apparent from a comparison of its requirementsof fuel, air and other utilities with those of a conventionalsolvent-based paint drying system in which the drying oven is operatedwith an atmosphere below the lower explosion limit of the gas mixtureformed in the oven during the drying operation instead of above theupper explosion limit thereof as in the process according to theinvention. For example, in a conventional system used for comparisonpurposes and including a fume incinerator, a preheat recuperator, and aheat transfer liquid medium for total heat recovery, the gas consumptionthereof is approximately 41,000,000 B.t.u. per hour, and the heattransfer rate to the work is approximately 7,000,000 B.t.u. per hour.For the same heat demand in the paint drying oven 10, a properlydesigned paint drying system according to the invention reduces the fuelconsumption to less than 5,000,000 B.t.u. per hour or only 12 percent asmuch fuel requirement. The fresh air requirements of the systemaccording to the invention are only 200,000 s.c.f.h. (standard cubicfeet per hour) or about 13 percent of the 1,500,000 s.c.f.g. required bythe conventional system. In addition, the lower fume volume ofapproximately 525,000 s.c.f.h. for the process comprising the inventioncompared to the 1,300,000 s.c.f.h. for the conventional system makes itpossible to employ smaller size exhaust fans, piping, and theincinerator itself, thus further effecting economies in the form ofreduced cost for the required equipment.

Insofar as the quality of the finished paint coating is concerned,comparative tests of thermosetting type paint samples indicate that thecomposition of the atmosphere in the drying oven 10 does not have anyeffect on the finished paint properties or color character istics. As amatter of fact, the inert atmosphere employed in the paint drying oven10 in accordance with the invention retains the paint qualities when thepaint is exposed to such inert atmosphere for a long time, whereas withhigh oxygen-containing atmospheres such as are employed in the dryingovens of conventional paint drying systems the paint pigments areoxidized and become darkened. Thus, if anything the drying ofsolvent-based paint coatings in inert gas atmospheres in accordance withthe invention actually results in better paint finishes and quality.

What is claimed is:

l. A method for drying solvent-based coatings on articles moving througha drying chamber by means of exhaust gases coming from a combustionapparatus having an intake in fluid communication with a gaseous exhaustduct in said drying chamber whereby a furnace atmosphere generatedwithin said drying chamber is drawn into said combustion apparatus, saidmethod comprising the Steps of:

injecting combustion air into said combustion apparatus at a fixed ratesufficient to mix stoichiometrically with said furnace atmosphere drawnfrom said exhaust duct when said drying chamber is operating underconstant load; initially mixing, during startup, a gaseous fuel from anauxiliary burner in stoichiometric proportions with said combustion air;igniting said gaseous fuel and said combustion air in said combustionapparatus by means of at least one pilot burner to produce an inert typegas mixture possessing not more than five percent oxygen by content;circulating at least a portion of said inert gas mixture leaving saidcombustion apparatus through a heat exchange mechanism to cool same to apredetermined value; piping at least a portion of said inert gas mixtureleaving said cooling mechanism into said drying chamber to purge same;introducing said articles through an article inlet end into said dryingchamber to produce a furnace atmosphere containing solvent emissions andremoving said articles through an article outlet end of said chamber;continuously removing a first portion of said furnace atmosphere throughsaid exhaust duct and burning said removed solvents in said combustionapparatus to continuously produce an inert type off-gas; sensing thetemperature of said offgas to regulate the flow of fuel to saidauxiliary burner to insure that said off-gas possesses no more than fivepercent oxygen by content; circulating a portion of said off-gas throughsaid heat exchange mechanism to cool same to a predeterminedtemperature; introducing a portion of said cooled off-gas to said dryingchamber to insure that said furnace atmosphere is outside the explosiverange; exhausting a second portion of said furnace atmosphere from saiddrying chamber through a vent; recirculating said second portion of saidfurnace atmosphere into said drying chamber after mixing same with saidportion of inert type off-gas being circulated into said drying chamber;controlling the flow of said off-gas to produce a slight overpressure insaid drying chamber; circulating a portion of said inert type off-gasdownstream from said heat exchange mechanism into a plenum arrangementat said article inlet end and said article outlet end; and ejecting saidinert type gas mixture from said plenum arrangement in a plurality ofgaseous streams directed generally normal to the flow of said articlesat a pressure sufficient to establish gas flow patterns through saidarticle inlet and outlet ends whereby said overpressure is maintainedand said furnace atmosphere is substantially sealed. 2. Apparatus fordrying solvent-based coatings on articles, said apparatus comprising:

a drying oven having a drying chamber, an article inlet opening, anarticle outlet opening, a gaseous inlet opening for injecting a furnacetype gaseous atmosphere into said oven, an exhaust outlet opening forremoving a first portion of the furnace atmosphere, and a vent forremoving a second portion of the furnace atmosphere;

combustion means in fluid communication with said gaseous exhaust outletfor burning furnace fumes exiting therefrom to produce an inert typeoff-gas mixture which contains not more than five percent oxygen bycontent;

said combustion means including combustion air supply means preset todeliver a fixed quantity of combustion air to said combustion means toachieve approximately stoichiometric combustion when said apparatus isoperated under constant load, pilot burner means to ignite saidcombustion air with said exhaust furnace fumes to produce said inerttype off-gas mixture, auxiliary fuel means effective during startup ofsaid apparatus to mix with said combustion air in said combustion meansto produce an inert type off-gas suitable for purging said dryingchamber and effective during normal operation of said apparatus tocontrol said oxygen content within said five percent limit in said inertoff-gas mixture, and temperature control means associated with saidcombustion means for controlling the fuel quantity supplied saidcombustion means by said auxiliary fuel means;

recycle conduit means for recycling at least a portion of said inert gasmixture from said combustion means to said gaseous inlet opening in saiddrying chamber;

said recycling means including a heat transfer mechanism for reducingthe temperature of said inert offgas mixture to a predetermined value,first conduit means upstream of said heat transfer mechanism for ventinga portion of said inert off-gas mixture directly to the atmosphere asnonpolluting emissions, second conduit means in fluid communication withsaid gaseous inlet of said drying chamber for conveying and mixing asecond portion of said inert type off-gas mixture with the solventsemitted from said coatings to produce a furnace gas atmosphere which isabove the upper explosive limits of said atmosphere at all times duringthe drying the said coatings;

recirculating means including a third conduit in fluid communicationwith said vent for circulating a second portion of said furnaceatmosphere with said inert type gaseous mixture whereby the heatsensibility of the furnace mixture is conserved and temperatureuniformity of the furnace atmosphere is promoted;

pressure means for maintaining said drying chamber at a slightoverpressure;

aerodynamic sealing means at said article inlet opening and said articleoutlet opening directing a plurality of streams of said inert typeoff-gas mixture in a direction generally normal to all surfaces of saidarticles at a flow rate sufficient to establish gas flow into saiddrying chamber to maintain said drying chamber substantially sealed fromthe outside atmosphere; and

said recycling means including a fourth conduit supplying said inert gasmixture to said aerodynamic seal means.

1. A method for drying solvent-based coatings on articles moving througha drying chamber by means of exhaust gases coming from a combustionapparatus having an intake in fluid communication with a gaseous exhaustduct in said drying chamber whereby a furnace atmosphere generatedwithin said drying chamber is drawn into said combustion apparatus, saidmethod comprising the steps of: injecting combustion air into saidcombustion apparatus at a fixed rate sufficient to mixstoichiometrically with said furnace atmosphere drawn from said exhaustduct when said drying chamber is operating under constant load;initially mixing, during startup, a gaseous fuel from an auxiliaryburner in stoichiometric proportions with said combustion air; ignitingsaid gaseous fuel and said combustion air in said combustion apparatusby means of at least one pilot burner to produce an inert type gasmixture possessing not more than five percent oxygen by content;circulating at least a portion of said inert gas mixture leaving saidcombustion apparatus through a heat exchange mechanism to cool same to apredetermined value; piping at least a portion of said inert gas mixtureleaving said cooling mechanism into said drying chamber to purge same;introducing said articles through an article inlet end into said dryingchamber to produce a furnace atmosphere containing solvent emissions andremoving said articles through an article outlet end of said chamber;continuously removing a first portion of said furnace atmosphere throughsaid exhaust duct and burning said removed solvents in said combustionapparatus to continuously produce an inert type off-gas; sensing thetemperature of said off-gas to regulate the flow of fuel to saidauxiliary burner to insure that said off-gas possesses no more than fivepercent oxygen by content; circulating a portion of said off-gas throughsaid heat exchange mechanism to cool same to a predeterminedtemperature; introducing a portion of said cooled off-gas to said dryingchamber to insure that said furnace atmosphere is outside the explosiverange; exhausting a second portion of said furnace atmosphere from saiddrying chamber through a vent; recirculating said second portion of saidfurnace atmosphere into said drying chamber after mixing same with saidportion of inert type off-gas being circulated into said drying chamber;controlling the flow of said off-gas to produce a slight overpressure insaid drying chamber; circulating a portion of said inert type off-gasdownstream from said heat exchange mechanism into a plenum arrangementat said article inlet end and said article outlet end; and ejecting saidinert type gas mixture from said plenum arrangement in a plurality ofgaseous streams directed generally normal to the flow of said articlesat a pressure sufficient to establish gas flow patterns through saidarticle inlet and outlet ends whereby said overpressure is maintainedand said furnace atmosphere is substantially sealed.
 2. Apparatus fordrying solvent-based coatings on articles, said apparatus comprising: adrying oven having a drying chamber, an article inlet opening, anarticle outlet opening, a gaseous inlet opening for injecting a furnacetype gaseous atmosphere into said oven, an exhaust outlet opening forremoving a first portion of the furnace atmosphere, and a vent forremoving a second portion of the furnace atmosphere; combustion means influid communication with said gaseous exhaust outlet for burning furnacefumes exiting therefrom to produce an inert type off-gas mixture whichcontains not more than five percent oxygen by content; said combustionmeans including combustion air supply means preset to deliver a fixedquantity of combustion air to said combustion means to achieveapproximately stoichiometric combustion when said apparatus is operatedunder constant load, pilot burner means to ignite said combustion airwith said exhaust furnace fumes to produce said inert type off-gasmixture, auxiliary fuel means effective during startup of said apparatusto mix with said combustion air in said combustion means to produce aninert type off-gas suitable for purging said drying chamber andeffective during normal operation of said apparatus to control saidoxygen content within said five percent limit in said inert off-gasmixture, and temperature control means associated with said combustionmeans for controlling the fuel quantity supplied said combustion meansby said auxiliary fuel means; recycle conduit means for recycling atleast a portion of said inert gas mixture from said combustioN means tosaid gaseous inlet opening in said drying chamber; said recycling meansincluding a heat transfer mechanism for reducing the temperature of saidinert off-gas mixture to a predetermined value, first conduit meansupstream of said heat transfer mechanism for venting a portion of saidinert off-gas mixture directly to the atmosphere as nonpollutingemissions, second conduit means in fluid communication with said gaseousinlet of said drying chamber for conveying and mixing a second portionof said inert type off-gas mixture with the solvents emitted from saidcoatings to produce a furnace gas atmosphere which is above the upperexplosive limits of said atmosphere at all times during the drying thesaid coatings; recirculating means including a third conduit in fluidcommunication with said vent for circulating a second portion of saidfurnace atmosphere with said inert type gaseous mixture whereby the heatsensibility of the furnace mixture is conserved and temperatureuniformity of the furnace atmosphere is promoted; pressure means formaintaining said drying chamber at a slight overpressure; aerodynamicsealing means at said article inlet opening and said article outletopening directing a plurality of streams of said inert type off-gasmixture in a direction generally normal to all surfaces of said articlesat a flow rate sufficient to establish gas flow into said drying chamberto maintain said drying chamber substantially sealed from the outsideatmosphere; and said recycling means including a fourth conduitsupplying said inert gas mixture to said aerodynamic seal means.